The present disclosure relates in general to semiconductor manufacturing technology, and more particularly, to a method for gating photomask contamination.
Photomasks, or reticles, are commonly used for photolithography in semiconductor manufacturing. Photomasks are typically made from very flat pieces of quartz or glass with an absorption layer of chromium deposited on one side. The absorption layer may include a pattern that is transferred onto a semiconductor wafer on BIM (Binary Mask) or PSM (Phase Shift Mask) masks during a photolithography process. While contamination of photomasks has always been a problem, high precision masks used in photolithography with wavelengths equal to or less than 248 nm, are particularly susceptible to contamination. One type of photomask contamination is referred to as haze contamination. Haze contamination is a precipitant formed from mask cleaning chemical residual, or impurities of the fab or tool environment cross exposure. For example, when a solution including ammonium (NH4) and sulfate (SO4) is used to clean a photomask, contamination becomes apparent when the photomask is exposed to a short wavelength UV light, such as 193 or 248 nm.
Several methods have been used to detect these and other types of contamination or precipitates on the photomask. Although the methods have been satisfactory for their intended purposes, the methods have not been satisfactory in all respects. For example, one method uses an optical inspection tool to scan an entire surface of the photomask to detect for contamination or precipitates on the photomask. The optical inspection tool may be based on light scanning techniques that utilize various types of light beams such as laser beams or electron beams. However, the optical inspection tool is expensive to own and operate, and scanning the entire surface of a photomask is time-consuming. Accordingly, only a limited number of photomasks can be inspected in a day and a frequency at which the photomask can be inspected is also limited with the optical inspection tool.
Therefore, what is needed is a method and apparatus for gating photomask contamination.